pH is considered an inferred value. This means that raw data (in this case potential as mV and temperature) are used to calculate pH with a known algorithm called the Nernst Equation.
It's important to quantify the following two correction factors before calculating pH:
- Offset takes into account variances in the observed potential as a result of the electrode condition, as well as any manufacturing tolerances of the pH-sensitive glass bulb or the junction. This value is the potential observed using pH 7.00 calibration buffer. With an ideal electrode at 25°C, the offset determined in pH 7.00 buffer should be 0 mV. However, this may never actually be observed. An acceptable range for the offset is ± 30 mV.
- Slope takes into account variances in the quality of the calibration buffers used. This value is represented by the change in potential over a change in pH (mV/pH). When measuring calibration buffers at 25°C with an ideal electrode, the theoretical slope is -59.16 mV/pH. The HOBO MX2501 data logger takes into account the effect of temperature and reports slope as a percentage of the ideal slope. An acceptable range for the slope is 85-100%.